A photoplethysmographic (PPG) signal can be measured by PPG systems to derive corresponding physiological signals (e.g., pulse rate). In a basic form, PPG systems can employ a light source or light emitter that injects light into the user's tissue, and a light detector to receive light that reflects and/or scatters and exits the tissue. The received light includes light with amplitude that is modulated as a result of pulsatile blood flow (i.e., “signal”) and parasitic, non-signal light with amplitude that can be modulated (i.e., “noise” or “artifacts”) and/or unmodulated (i.e., DC). However, in some examples, the reflected and/or scattered light received by the light detector may be have a low signal strength, making it difficult to accurately determine the user's pulse rate.
One way to increase the signal intensity or signal strength can be to decrease the distance between the light sensor and light emitter. The minimum distance between the light sensor and light emitter can, however, be limited by mechanical or functional requirements of other components on the PPG system, such as the windows used to cover and protect the light source and light detector. An alternative way to increase the signal strength may be needed.